Project Summary. Breast cancer is among the most common forms of cancer among women. Adjuvant therapies that target estrogen receptor (ER) functions have proven to be efficacious for many patients; however resistance to these drugs occurs with some frequency. The long-term objective of this study is to develop an understanding of how overexpression of one protein, Breast Cancer Anti-Estrogen Resistance-3 (BCAR3), induces anti-estrogen resistance in ER+ breast cancer cells. We hypothesize that BCAR3 confers resistance through its ability to physically and functionally interact with other signaling proteins involved in regulating cell growth and survival. Two specific aims are proposed to test this hypothesis. The first is designed to investigate whether interactions between BCAR3 and one of its binding partners, Cas (also known as Breast Cancer Anti-Estrogen Resistance-1; BCAR1), are important for conferring resistance to the anti-estrogen Tamoxifen or the pure anti-estrogen ICI 182,780 (Fulvestant). Since overexpressionof Cas is also associated with anti-estrogen resistance in breast cancer cells as well as in patient tumors, it is possible that these molecules may coordinately regulate pathways that promote growth in the presence of anti-estrogens. Second, the molecular mechanisms contributing to anti-estrogen resistance that are activated downstream of BCAR3 signaling networks will be elucidated. Breast cancer cell lines engineered to express reduced, normal, or elevated levels of BCAR3 and/or Cas will be used for both biochemical and fluorescent microscopic analyses designed to investigate the molecular basis for BCARS-dependent antiestrogen resistance. Strategies will also be used to disrupt heterologous protein complexes and functional interactions to further understand these pathways. Finally, the effect of these manipulations on the therapeutic response of tumors in mice will be evaluated. Relevance. This project will provide insight into the nature of a molecular signaling pathway that is involved in rendering breast tumor cells resistant to anti-estrogens. These and related drugs are currently used to treat numerous women suffering from breast cancer, but resistance to these drugs is a major clinical problem in the treatment and management of this disease. It is anticipated that this study will lead to the identification of new molecular markers that can predict the response of individual tumors to Tamoxifen or Fulvestant. In addition, by understanding the mechanisms responsible for resistance, it may be possible to develop new targeted therapies that can render perviously resistant tumors sensitive to these drugs.